1. Field of the Invention
The present invention is related generally to a monolithically integrated waveguide-photodiode combination for integration into optical communications systems.
2. Description of the Related Art
In a publication by Stillman et al., "Monolithic Integrated In.sub.x Ga.sub.1-x As Schottky-Barrier Waveguide Photodetector", Applied Physics Letters 25, 36-38 (1974) and in a publication by Ostrowsky et al., "Integrated Optical Photodetector", Applied Physics Letters 22, 463-464 (1973), waveguide photodiode combinations are disclosed wherein incident radiation is conducted directly onto the photodiode by butt-coupling between the waveguide and the photodiode.
A European published application number 0 187 979 discloses a waveguide-photodiode combination wherein one component of the photodiode is composed of a material that has a higher refractive index than the material of the waveguide. Consequently, incident radiation is coupled into the photodiode. A waveguide layer composed of a quaternary material, for example InGaAsP, is applied to a substrate body of InP that either has an adequately smooth structured surface or an additional InP layer epitaxially applied. A photodiode is composed of a ternary material of, for example, InGaAs, and is formed on the layer waveguide. The photodiode is composed either of two layers epitaxially applied in succession where one layer is n-doped like the rest of the component and the other layer situated thereabove exhibits a p-doping, or the photodiode is composed of a single layer that is originally n-doped and into which a p-doping has been introduced from above, for example, by diffusion. The arrangement can be covered by a layer of Si.sub.3 N.sub.4, polyimide, or InP.
In the disclosed waveguide-photodiode combination, the photodiode is situated at the surface of the component in a mesa structure. This structure is characterized by a relatively high dark current for the photodiode, which results in poor documentation sensitivity when integrated in optical receivers. Moreover, the long-term stability of planar photodiodes is significantly higher than that of photodiodes having a mesa structure.